2748. Number of Beautiful Pairs

class Solution {
    public int countBeautifulPairs(int[] nums) {
        int pairs = 0;

        int[] prefixCountMap = new int[10];

        for (int num : nums) {
            for (int i = 1; i < 10; i++) {
                if (prefixCountMap[i] > 0 && gcd(i, num % 10) == 1) {
                    pairs += prefixCountMap[i];
                }
            }

            prefixCountMap[getFirstDigit(num)]++;
        }

        return pairs;
    }

    private int getFirstDigit(int num) {
        while (num >= 10) {
            num /= 10;
        }
        return num;
    }

    private int gcd(int a, int b) {
        return b == 0 ? a : gcd(b, a % b);
    }
}

References

2748. Number of Beautiful Pairs